1. Field of the Invention
The present invention relates to a magnetic transducer device for detecting coded magnetic information and to the method of producing such a device.
It is applicable in particular to devices for reading coded magnetic information such as check or card reading devices and magnetic tape units, etc.; however, for the sake of simplicity the invention will be described as it applied to a check reading device, with the understanding that the description is equally applicable to any other device for reading coded magnetic information.
2. Description of the Prior Art
It is known that present day data processing systems often include data infeed arrangements which employ carriers or slips bearing coded information. These carriers may take various forms, such as, for example, bank checks, post office checks, deposit or withdrawal slips, identification cards or credit cards, etc. In case of credit cards, this information frequently is contained in a strip across the front or back face of the card.
The information generally consists of a sequence of alpha-numeric characters printed on the slips, that is a sequence of letters of the alphabet, figures, punctuation marks, etc. which indicate, in the case where the slip is a check, for example, the number of the check or the account number of its drawer. Each character is formed by a set of bars composed of magnetic ink deposited on an insulating substrate. The number of bars, the distance between the bars, and their relative disposition are individual to each character and conform to known codes such as the CMC 7 code, for example. These bars are magnetized before being processed in the code reader.
Consideration will be given to a check and the corresponding data infeed device which is called a "check reader" as an illustrative example of an application to which the invention may be adapted. The check reader converts the coded magnetic information represented by the characters printed on the check into a succession of electrical signals. These electrical signals are translated to electronic shaping circuits which convert this series of electrical signals into a series of square-wave electrical pulses which are in turn transmitted to electronic circuits for recognizing the characters printed on the check. As soon as the pulses which correspond to this series of square-wave electrical pulses, which in turn correspond to the printed characters, have been decoded, that is to say the characters have been identified, it is possible to cause a calculating unit in the data processing system to which the check reader belongs to perform operations relating to the check such as debiting, crediting, updating the account of its drawer, etc.
In order that the object of the invention may be better understood, the following facts about magnetism should be noted:
To magnetize a magnetic material, the material is first subjected to a positive magnetic field whose strength is sufficient to saturate the material, that is to say, for the magnetic induction in the material to reach a limiting value B.sub.s as soon as the strength of the magnetic field reaches a certain value H.sub.s. The magnetic field is then removed. There then remains a magnetic induction termed the residual induction (+Mr) which is other than zero and which is characteristic of the material. In other words, magnetizing a magnetic material amounts to saturating it magnetically.
A magnetic material which has been magnetized sets up a magnetic leakage field H in the immediate vicinity of its surface, and the magnetic flux of a magnetic field H through an area S is equal to the product of the strength of this field multiplied by the size of the area.
Generally, check readers comprise a magnetizing device and a magnetic transducer device. The magnetizing device is used to magnetize the bars forming the characters printed on the check in order to render the value and sense of the magnetic induction identical in all the bars since this is necessary because when the characters are printed on the check, either the induction in the bars is zero or else the value and sense of the magnetic induction varies from one bar to the next throughout the bars. Thus, the magnetic induction in the bars is equal to the residual induction of the magnetic ink when they are no longer subject to the magnetic field of the magnetizing device.
The magnetic transducer device is sensitive to the magnetic leakage field which is set up by the bars after they have been magnetized by the magnetizing device and emits an electrical signal in response to this magnetic leakage field. This signal is transmitted to the aforementioned electronic shaping circuits. In other words, it can be said that the magnetic transducer device enables the presence of the bars to be detected.
The check is moved by a mechanical transporting device and is positioned in the device in such a way that all the bars pass in front of the magnetizing device and in front of the magnetic transducer device in succession in close proximity thereto. The mechanical check transporter device may be operated either manually or by an electronic motor.
In present day practice, magnetic transducer devices are formed by a head consisting of a magnetic circuit which is provided with a wide air gap and around which is coiled a winding containing a large number of turns. The bars pass by the air gap at a very short distance from it so that it picks up the major proportion of the magnetic flux which is generated by the magnetic leakage field of the bars, via the magnetic circuit of the head. An electrical signal is then received at the terminals of the winding whose voltage is equal in absolute value to the variation, per unit of time, of the magnetic flux picked up by the magnetic circuit. In can be shown that this voltage is proportional to the speed of movement of the bars past the air gap. It follows that this voltage is sensitive on the one hand to the speed of movement and on the other hand to variations in it (for example, in the case of a check transporter device which is manually operated), which makes for inaccurate detection of the bars. Such a magnetic head needs care in manufacture and is relatively expensive and bulky.
Magnetic transducer devices having at least one magnetoresistor overcome these disadvantages. Magnetoresistors are electrical resistors which are deposited on a substrate of insulating material in the form of thin layers or films of very shallow depth (a few hundred Angstroms to a few microns thick) and whose resistance varies when they are subjected to the flux of a magnetic field. A measuring magnetoresistor R of this kind is connected to the terminals of a generator which ouputs a current I. When a bar passes in front of the magnetoresistor of the flux of the magnetic leakage field H causes a change .DELTA.R in its resistance and thus a change .DELTA.V=I.DELTA.R in voltage, which means that .DELTA.V/V=.DELTA.R/R, .DELTA.R/R being termed the coefficient of magnecoresistance. This coefficient is generally a few percent and is very often negative.
The corresponding electrical signal is amplified and transmitted to the aforementioned shaping circuits. This signal is unaffected by the speed of movement of the bars.
In present day practice, magnetoresistive magnetic transducer devices include, for preference, two magnetoresistors for detecting the presence of bars which are deposited on a single insulating substrate and which are positioned a short distance d apart, the bars passing in front of each of the magnetoresistors in succession.
The distance d depends in particular on the width of the bars and the maximum and minimum allowable spacing between them. Such a device is described, for example, in an article by G. E. Moore, Jr. and Lijcote entitled "Dual Strips Magnetoresistive Read Heads for Speed-Insensitive Tape Readers" which was published in the journal "IEEE Transactions of Magnetics," Vol. 12, Number 6 of November 1976.
Such magnetoresistive devices have the drawback of being extremely sensitive to magnetic and electromagnetic fields other than the magnetic leakage field H of the bars. Such fields will be referred to generally in what follows as magnetic interference fields. Among them may be mentioned the magnetic fields which are set up by electrical apparatus of all kinds and the magnetic field of the earth.
Even if these fields are weak and are less than the magnetic leakage field H of the bars, their effect on the magnetoresistors for detecting the presence of the bars is nevertheless to disturb the electrical output signal from the resistors due to the field H by generating by no means negligible electrical signals termed "noise" signals. In other words, the overall electrical output signal from the magnetoresistors is formed by the electrical "noise" signals due to the interference fields, superimposed on the signal due to the leakage field H of the bars. As a result there may be a considerable risk of errors occurring in detecting the presence of the bars.